SIB J. LUBBOOK ON THE OllIGHN OP INSECTS. 425 



IS true that in CJiloeon the vibration of the so-called branchiae is 

 scarcely, if at all, utilized for the purpose of locomotion ; the 

 branchiaa are, in fact, placed too far back to act efficiently. The 

 situation, however, of these branchiae differs in different groups ; 

 indeed it seems probable that originally there would be a pair 

 ou each segment. In such a case those branchiae which were 

 situated near the centre of the body, neither too much in front 

 nor too far back, would serve the most efficiently as propellers. 

 The same causes which have determined the position of the legs 

 would affect the wings also. Thus a division of labour would be 

 effected ; tiie branchiro on the posterior segments of the thorax 

 would be devoted to locomotion, those on the abdomen to respi- 

 ration. This would tend to increase the development of the tho- 

 racic segments, already somewhat enlarged to receive the muscles 

 of the legs. 



That wings may be of use to insects under water is proved by 

 the very interesting case of Folijnema natans, which I discovered 

 in 18G2, and which uses its wings to swim with. This, however, 

 is a rare case ; and it is possible that the principal use of the 

 wings was, primordially, to enable the mature forms to pass from 

 pond to pond, thus securing fresh habitats and perhaps avoiding 

 in-and-in breeding. If so, the development of wings would tend 

 to be relegated to a late period of life; and by the tendency to 

 the inheritance of characters at corresponding ages, to which 

 Mr. Darwin has called attention*, the development of wings 

 would be associated with the maturity of the insect. Thus the 

 late acquisition of wings in the Insecta generally seems to be itself 

 an indication of their descent from a stock which was at one 

 period aquatic in its habits, and which probably resembled the 

 present larva) of Gliloeon in form, but had thoracic as well as ab- 

 domiual branchia). 



If these views are correct, the genus Gampodea must be re- 

 garded as a form of remarkable interest, since it is the living re- 

 presentative of a primaeval type from which not only the CoUembola 

 and Thysanura, but the other great orders of insects have all 

 derived their origin. 



* Origin of Species, 4th ed. pp. 14 & 97. 

 LINN. JOUBN, — ZOOLOGY, VOL. XI. 31 



